The present invention relates to precision boring tools for internal machining of a bore, and more specifically the invention relates to a cutting tool that has improved cutting properties and cutting lifetime when precision machining stainless steel and equivalent material to very fine tolerances.
The cutting tool in an ultra-precision machining of stainless steel must have a capability of stable cutting and keeping tight tolerances, providing surface finish and roundness on a work piece during the tools time life, and at the same time provide the required output. One way to increase output is to increase the cutting conditions, that is, the cutting speed and the feed rate. An increase in cutting speed will decrease the tools lifetime due to the increased cutting temperature, and the build-up on the cutting surface of the tool. Two potential approaches to solving this problem should be considered: a more efficient design for the cutting nose of the tool, and a better coating for the nose.
In order to protect a tool""s working surface, different types of coating have been applied. This coating can be single or multiple layers applied as a PVD/CVD coating. Several coating types have been recommended in the past, including a single layer PVD TiN coating for most stainless steels at low to moderate cutting speed, or three layers comprising TiN, TiCN, and TiN applied as a CVD/PVD coating for semi-finishing to finishing on austenitic stainless steels. Alternatively, multiple layers of TiN, AL203, and TiN/TiCN is proposed for general purpose to high speed cutting of stainless steel; another proposal for cutting stainless steel comprises four layers, including TiN, TiCN, TiC, and TiAlN.
However, uncoated tools have a smaller radius of cutting edge, and greater edge toughness than a coated tool. Coated tools do not work well on interrupted cuts and on cuts of varying depth due to mechanical and thermal shocks as well as fatigue. This has become a serious problem of ultra-precision machining, such as is required for making the bore in the sleeve of a hydrodynamic bearing and the like; this is especially true for the finish pass when the cutting process does the burnishing and chattering which will effect the part""s roundness and surface finish. For these reasons, most companies do not use a coating on a cutting tool that is to be used for finishing of stainless steel.
Tests with Boring tools with a TiN single layer coating have established that this coating does not have a good adhesion to substrate, has a droplet phase, and does not prevent buildup on the cutting corner of the tool. It appears that the droplet phase or micro particles are present due to cathodic arc deposition method of applying the coating. This increases the roughness of the cutting surface, dulls sharp cutting edges, and does not have reliable adhesion to the substrate. The multilayer coatings described above are usable for rough and semifinish cutting, but not for the ultra smooth finish needed for the base of a hydrodynamic bearing due to the increase in the radius of the cutting edge, and inevitable coat chipping.
As to increasing the feed rate, a primary way to increase the feed rate and keep the required surface finish is to increase the tool nose radius. The proper nose radius is one of the most important factors when a specific surface finish and roundness are required. However, the nose radius is usually limited by the work piece geometry. That is the necessary radii or fillets which must be defined. The other limitation increasing tool nose radius is bore roundness which depends on tool deflection under thrust force. Cutting force measurements have established that with increasing tool nose radius from 50 xcexcm to 250 xcexcm the thrust force increases about three times, and the cutting process has a tendency toward chattering. Therefore, the problems with optimizing cutting conditions for a hydrodynamic bearing bore by optimizing cutting speed and feed rate remain.
In summary, the cutting tool for machining stainless steel, according to this invention is designed to provide both an increase cutting speed and an increase in feed rate. As a modification of the known round nose geometry, according to the present invention, a tool nose geometry with several facets is proposed.
Further, instead of the cathodic arc deposition method of TIN coating, a hollow cathode discharge method of TiCN coating with no more than three micron thickness is proposed. This coating method creates a high density of plasma vapor, does not contain micro particles, produces a very dense film which completely replicates the finish of the surface being coated, prevents buildup and increases tool performance.
These and other objectives of the present invention are achieved providing a tool nose geometry which has multiple facets. Specifically, a tool is provided including a nose comprising a leading edge which chips and removes stock from the edge surface, the leading edge being adjacent a calibration edge which thereby calibrates the bore surface as the tool is fed and the sleeve is rotated to form the bore opening. In a preferred embodiment the calibration edge or facet has a length equal to the feed per revolution. This same side has a cutting angle length equal to zero. The combination of these requirements provides an ideal surface, as the impression formed on the work piece by the nose of the cutting tool is a mirror image of the edge EG, provided minimum deflection of the work piece or tool occurs.
The leading facet preferably has a cutting edge angle with the direction of feed of the tool through the bore equal to 45xc2x0 which makes the cutting process smooth, and provides strengthening of the point which is common to the end cutting edge and the cutting surface. Further, appropriate relief angles are provided in the cutting surface, the leading nose edge, and the upper nose geometry to provide sufficient strength to the nose of the tool, and to further provide that chipping and deflection are minimized in the leading edge of the tool so that the machine bore of the central bore can occur in a single pass of the tool.
Other features and advantages of the present invention would become apparent to a person of skill in the art who studies the present invention disclosure given with respect to the following figures.